Update troubleshooting-upgrades.md

[stefnestor]

Summary

👋 Adding back in a variation of an earlier sentence that got moved to main upgrade doc (#6396, #5848).

During a rolling upgrade, primary shards assigned to a node running the new version cannot have their replicas assigned to a node with the old version. The new version might have a different data format that is not understood by the old version.

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🔍 Preview links for changed docs

• troubleshoot/elasticsearch/troubleshooting-upgrades.md

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✅ Elastic Docs Style Checker (Vale)

No issues found on modified lines!

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Docs review summary

Focus areas

• Style and clarity: Two small grammar issues in the new/changed text — a subject-verb agreement slip in the new list item and a missing article in the rewritten shard-allocation explanation. See inline comments.
• Jargon: No unexplained Elastic-internal jargon in the changed lines.
• Frontmatter and applies_to: Unchanged by this PR; no issues introduced.
• Content type fit: Troubleshooting content type is preserved; the added sentence and list item fit the existing Symptoms/Resolution structure.
• Contradictions: No contradictions found.
• Parent issue satisfaction: Not applicable — no linked parent issue found.

Nits

• Vale flagged "for the duration of" (line 19) and a semicolon (line 149) as pre-existing wordiness/style suggestions, but neither is part of this PR's diff.

Notes

• Review scope covered only the single eligible changed markdown file, troubleshoot/elasticsearch/troubleshooting-upgrades.md.

Generated by Docs review agent for #8051 · sonnet50 · 35.9 AIC · ⌖ 3.9 AIC · ⊞ 17.7K

[github-actions]

"nodes on later version" is missing an article. Use "on the later version" for consistency with the rest of the page, which always pairs "the" with "later version" and "earlier version".

Suggested change

	Primary shards assigned to a node running the later version cannot have their replicas assigned to nodes with the older version. If replica shard allocations are blocked due to insufficient eligible nodes on later version, continue upgrading your nodes to resolve the preceding errors.
	Primary shards assigned to a node running the later version cannot have their replicas assigned to nodes with the older version. If replica shard allocations are blocked due to insufficient eligible nodes on the later version, continue upgrading your nodes to resolve the preceding errors.